This invention relates to processes for crosslinking melt-processed polymer compositions and to compositions crosslinkable thereby.
To avoid environmental pollution, manufacturing processes that do not require the use of volatile solvents have become of great interest. Hot-melt coating techniques have developed in response to this need. Unlike solvent coating techniques, where a polymer composition is simultaneously coated and crosslinked, hot-melt coating requires that coating and crosslinking be performed sequentially. This requirement is due to competing considerations, as a polymer composition that is already crosslinked cannot be hot-melt coated effectively, yet a polymer composition needs to be crosslinked in order to achieve certain desirable performance characteristics (for example, high peel and shear adhesion characteristics, if the polymer composition is a pressure-sensitive adhesive (PSA) composition). Thus, hot-melt coating is performed first, followed by crosslinking of the resulting coated polymer composition.
Since hot-melt coating involves the use of large amounts of thermal energy and shear, thermal crosslinking techniques (for example, using non-copolymerized blocked isocyanates) have generally not been sufficiently controllable to achieve wide use, and thus non-thermal crosslinking techniques have generally been utilized. The traditional approaches have involved exposure to high energy radiation, for example, electron beam (E-beam) or high intensity ultraviolet (UV) radiation. E-beam techniques, however, have required high capital investment and have been complex to control. Accordingly, much interest has been focused on UV radiation techniques.
UV radiation crosslinking of coated polymers has relied almost exclusively on hydrogen abstraction methods, in which a hydrogen abstracting agent (for example, benzophenone or anthraquinone) is mixed into the polymer composition prior to or during the coating process, and the resulting composition is then crosslinked by exposure to UV radiation. The separate mixing step can be eliminated by incorporating the hydrogen abstracting functionality directly into the polymer, for example, by copolymerization (with other monomers) of an unsaturated monomer containing a hydrogen abstracting moiety. Although this latter technique provides a more efficient crosslinking mechanism, a substantial amount of high energy UV exposure is necessary to achieve a satisfactory degree of crosslinking, as the energy necessary to activate the hydrogen abstracting moieties is relatively high.
This limitation is especially pronounced for thick polymer coatings because of the difficulty in achieving deep penetration of high levels of UV radiation. A crosslinking gradient therefore often results, especially when the polymer (or any additives present) absorb near the absorption maximum of the hydrogen abstracting moieties. Furthermore, neither foaming nor the use of opaque fillers or pigments is possible (due to interference with UV penetration), and, since any compound with an abstractable hydrogen atom can be involved in the crosslinking reaction, many common tackifiers, plasticizers, antioxidants, and other common additives cannot be utilized.
Thus, there remains a need in the art for a process for crosslinking melt-processed polymer compositions that does not require expensive equipment, that can be easily and effectively adjusted and controlled to obtain the desired level of crosslinking, that can be used to crosslink even foamed or filled compositions and thick polymer coatings without producing a crosslinking gradient, and that is effective even in the presence of common additives.
Briefly, in one aspect, this invention provides a process for crosslinking a melt-processed polymer composition comprising the steps of (a) forming a polymerizable mixture comprising (i) at least one monoethylenically-unsaturated monomer or prepolymer and (ii) at least one monoethylenically-unsaturated, blocked mono- or polyisocyanate that is thermally stable at a selected melt processing temperature range and that is thermally deblockable at and above a deblocking temperature that is higher than the selected melt processing temperature range; (b) exposing the polymerizable mixture to transmissive energy to polymerize the mixture to form a polymer composition comprising polymerized units derived from the blocked mono- or polyisocyanate; (c) melt processing the polymer composition at the selected melt processing temperature range to form a melt-processed polymer composition; and (d) during melt processing or thereafter, increasing the temperature of the melt-processed polymer composition to the deblocking temperature, in the presence of at least one isocyanate-reactive crosslinking agent, to effect crosslinking of the melt-processed polymer composition. Preferably, the monomer is an acrylate or methacrylate monomer, and the blocked mono- or polyisocyanate, upon deblocking, releases a compound having a boiling point below the deblocking temperature.
It has been discovered that, in spite of the large amounts of thermal energy involved in the melt processing of polymer compositions, thermal techniques for crosslinking such compositions need not be totally overlooked. Surprisingly, the thermal deblocking of blocked isocyanates can be controlled so as to prevent premature crosslinking during melt processing, yet enable reliably fast and even crosslinking to occur upon demand thereafter. Control can be achieved by using blocked isocyanates that are copolymerizable, that are thermally stable at the selected melt processing temperatures, that are thermally deblockable at and above a deblocking temperature that is higher than the selected melt processing temperatures, and that preferably deblock to release a compound (the blocking agent or a residue thereof) that is volatile at the deblocking temperature. Such blocked isocyanates can withstand typical melt processing temperatures without volatilization and loss (since they, upon copolymerization with the other monomer(s), become an integral part of the polymer itself), and they do not significantly deblock at the melt processing temperatures (since their deblocking temperatures are higher). However, when the temperature of the polymer composition is raised to the deblocking temperature, crosslinking occurs quickly (apparently due to a rapid and substantially complete deblocking of the isocyanate, which appears to be facilitated by the release of volatile blocking agent).
Thus, the deblocking step of the process of the invention can be carried out, for example, in a later stage of the melt processing step (for example, by raising the temperature of the last zone (adjacent to the coating head) of a melt extruder) or after melt processing (for example, by using inexpensive, readily available equipment such as infrared heaters or ovens for raising the temperature of the melt-processed composition). Only short heat (infrared) exposure times are needed to effect the crosslinking of even thick polymer coatings, with essentially no limitations as to foaming, pigmentation, tackifier use, etc.
Thus, the process of the invention meets the need in the art for a process for crosslinking melt-processed polymer compositions that does not require expensive equipment, that can be easily and effectively adjusted and controlled to obtain the desired level of crosslinking, that can be used to crosslink even foamed or filled compositions and thick polymer coatings without producing a crosslinking gradient, and that is effective even in the presence of common additives.
In another aspect, this invention also provides a pre-PSA composition comprising (a) at least one acrylic or methacrylic ester of a non-tertiary alkyl alcohol, the alkyl group having from 1 to about 20 carbon atoms; (b) at least one monoethylenically-unsaturated, blocked mono- or polyisocyanate that is thermally stable at a selected melt processing temperature range and that is thermally deblockable at and above a deblocking temperature that is higher than the selected melt processing temperature range; and (c) at least one isocyanate-reactive crosslinking agent. Preferably, the blocked mono- or polyisocyanate, upon deblocking, releases a compound having a boiling point below the deblocking temperature. Upon polymerization of the pre-PSA composition, the resulting polymer composition is melt-processable, yet thermally-crosslinkable.
In other aspects, this invention further provides a composition prepared by the process of the invention, an article comprising the composition, a PSA composition prepared from the pre-PSA composition of the invention, and an article comprising the PSA composition.